Generally, when we've seen these issues, it means there's a corrupted GCF somewhere even if steam doesn't say there is one. Currently, the only known solution is to close steam entirely and delete:
All sin episodes GCF files
The ClientRegistry.blob file in steam's directory
and then re-downloading the entire game through steam. Yes, this is a monumental pain and we wish it wasn't necessary, but this has fixed most of the random lock-up issues we've seen so far. We hope to have another fix for this issue available soon, but this is currently the supported solution.